why cant the group take out down the river?

Because Tom has been shot.

"Once safe, they proceed to where the raft is hidden and Tom tells Jim he is a free man again, and that he will always be a free man from now on. Jim thanks him and tells him it was a great escape plan. Tom then shows them where he got a bullet in the leg, but Jim is worried for Tom's health. Jim rips up one of the Duke's old shirts and ties up the leg with it."